Lighting devices of this kind are incorporated into the external or internal rear view mirrors of motor vehicles and serve either to illuminate the interior or to illuminate the road surface in the entry region in front of the vehicle's doors. The light generated by the light source emerges at a mirror housing opening and radiates therefrom in beam form. The generated luminous intensity in the various regions of the field of illumination thus substantially depends on the distance between the region in question and the lighting device and on the angle of the incident light.
Since the installation location of the lighting device is predetermined by the positioning of the external or internal rear view mirror, in many cases the luminous intensity is distributed in the field of illumination in a manner which is not optimal. Far distant places in particular are only relatively poorly lit, for which reason a high illumination power is required. It is therefore known to arrange optical components in the path of the beam of the lighting device by means of which the distribution of the luminous intensity may be influenced. Fresnel lenses in particular have been proposed for this application because they are inexpensive to produce and are of compact size. Fresnel lenses are plate-like light refraction elements, of glass or plastic for example, which at least on one side have a surface structure by means of which the Fresnel lens is divided up into several light refraction zones. The light refraction zones correspond to circular ring-shaped lens sections which are pushed together stepwise in the plate plane so that the light refraction in a Fresnel lens approximately corresponds to the light refraction in a three-dimensional lens body.
It is a drawback to the use of the known centric Fresnel lenses that the light beam can only be scattered or collected depending on design, by the refraction at the Fresnel lens. This means that the light beam generated can be either widened or concentrated by the Fresnel lens.
A deflection of the light beam into a certain preferred direction cannot be achieved by the centric Fresnel lens as the light refraction zones are arranged circular-symmetrical to the optical center line.